I went to the doctor and told him I had trouble remembering things.

This classic doctor joke gets its laugh from a brilliant bit of ironic wordplay and a perfectly executed comedic misdirection (Review). The humor mechanism here is the patient's immediate and undeniable demonstration of the very symptom they are complaining about. The punchline directly illustrates their memory problem, creating a humorous, self-referential loop that catches the listener off guard. It's a clever way to show, not just tell, the core issue.

In the real world, memory loss is a common concern that brings people to their doctors, and one of the first questions a physician will often ask is about the onset and duration of symptoms. This standard medical inquiry provides the perfect setup for the joke. The patient's inability to recall when their forgetfulness began isn't just a symptom; it becomes the punchline itself, brilliantly subverting the doctor's diagnostic process and delivering a hearty chuckle.